Pipe Model for heat transfer (Conjugate heat transfer)

Dear All,

I solving the conjugate heat transfer problem.

The problem definition is:

In a shell and tube configuration, the heat transfer fluid is flowing in tube with some velocity and shell side is phase change material. The length size of configuration is very large compared with its inner and outer diameter.

The heat transfer fluid is flowing through the inner pipe and the shell side is phase change material.

Now I would like to used convective heat flux boundary at the exit condition. So can anybody tell me which expression or equation I have to used for solving the convective heat flux boundary at exit.

Thanking you in advance.

Vinay Patel